The Climate, Life, and Landscape of Jurassic
Time
The geologists who attempt to reconstruct the geography and climate
of the Jurassic Period first gather all possible facts and try to fit
them together to form a logical pattern. The results are then examined
for weak points and an attempt is made to find field evidence throwing
light on these weak points. The final result represents the sum of our
knowledge at the time but is subject to change as new facts are
obtained. Thus the following outline represents present thinking that
may be changed somewhat by future studies.
The land for miles around the Dinosaur Quarry was a low-lying desert
in early Jurassic time. The mountains you see now had not yet been
formed, and the whole desert area lay close to sea level. Great restless
sand dunes drifted across this level land to form a blanket 700 feet
thick. As the earth's crust sank, these dunes were covered by a long arm
of an arctic sea that extended southward along the present trend of the
Rocky Mountains across Canada, Montana, Wyoming, and Utah. Millions of
years later, in late Jurassic time, when the Morrison formation was
deposited, the area rose again and the stage was set for the
dinosaurs.
Imagine if you can, the vast plains extending from Mexico to Canada
and from central Utah to the Mississippi River. To the west were high
mountains in the Great Basin region of Nevada and western Utah. From
these highlands flowed great sluggish streams that carried large amounts
of sand and silt. Since the plains were almost flat, swamps and small
lakes were probably numerous. The streams may have changed their courses
from time to time as they were not confined to deep valleys. When the
whole region emerged from the sea the climate became more humid.
Volcanoes were active for to the west; the winds carried clouds of
ejected dust eastward and deposited them on the plains. Semi-tropical
conditions probably existed throughout the United States and in parts of
Canada. These deposits are called the Morrison formation.
The warm humid climate provided ideal conditions for plant growth.
Great forests of lush vegetation covered the land. Many of these plants
have since disappeared, but some of their related species may be found
today in the tropics. Most of the plants of our Temperate Zone had not
yet evolved. However, there were tall stands of a type of pine, and
other evergreens. There were also gingkos and curious tree ferns.
Various herb ferns formed a ground cover as thick and lush as grass
on a well-watered prairie. Palmlike ferns resembling today's cycads were
common, while along the river grew horsetail rushes like those living
today. Flowering plants of the Recent Epoch of geologic time (in which
we are now living) had not yet made their appearance. Thus the hardwood,
broadleaf forests of oak, elm, beech, maple, and similar trees were
absent. So too were the flowering shrubs familiar to the Temperate Zone.
Even the grasses were missing.
However, if you could picnic in this strange plant world you would
soon be slapping mosquitos and cursing the ants. For even in such
ancient times these insects were present; and so were a great variety of
other insects as is known from the more than 1,000 species that have
been discovered in Jurassic rocks. Among them were representatives of
most modern orders such as grasshoppers, beetles, moths, ants, and
flies. Jurassic insects probably looked much like those of today.
Among the most interesting of the strange reptiles were the
pterosaurs that dominated the skies. They resembled the modern bats in
some ways but their leathery wings were supported on each side by one
finger instead of four, and their skins were scaly or bare instead of
hairy. Some forms had long tails that were flattened at the tip and
helped them balance in flight, but others were tailless. Some pterosaurs
were no larger than sparrows while others had wing spans of 3 to 4
feet.
Crocodiles sunned themselves on the banks of sluggish streams and
lakes. They probably looked a good deal like those that live in modern
swamps and their habits were similar. Many a small dinosaur fell victim
to their stealthy attack and disappeared beneath the waters of some
ancient stream.
Birds have been found in Upper Jurassic rocks of Germany and may have
lived here too. Their fossil remains would probably have been classed as
reptiles had not feather imprints been a part of them. About the size of
crows, these reptile-like birds had small conical teeth, three-clawed
fingers on each wing, and a long tail instead of the fan of feathers
seen on modern birds.
Small mammals were also living at the time the Morrison formation was
being deposited and their remains have been found in the dinosaur quarry
at Como Bluff, Wyo. The largest ones were about the size of a house cat
but the majority were much smaller, probably about the size of today's
mice.
We do not know much about the habits of these early mammals but they
were probably rather shy and retiring. This would be expected in the
world of giants where they lived. Some of them lived in trees and there
was one group whose skull characteristics resemble those of the rodents.
It is likely that these primitive mammals lived a life similar to that
of the rodents millions of years later.
This, then, was the setting, the stage upon which the dinosaurs
played their leading roles. Although we have restricted our discussion
to Morrison time in northeastern Utah, the same or similar animals lived
all over the world. Worldwide humid and mild climates produced a
similarity of plant and animal life during most of the Mesozoic Era
whose like has not been seen in the last 60 million years. It was a
strange world and ruled by strange animals, but it must have been an
interesting one.
HOW DO WE KNOW?
This is a good time to stop and try to explain that this story, of
plants and animals of the past, has a firm foundation in today's
factsit is nor a fantasy.
The methods by which geologists and paleontologists have established
the age, climate, and life of Morrison time cannot be described for you
here in detail. To attempt such a description would require too much
space and would probably seem dull to most readers. Perhaps the best
approach is to describe some features and explain how they contribute to
our knowledge.
The rocks that were deposited here in Morrison time tell us much of
the story. The sandstones were once stream sandbars or perhaps beaches
around lakes. The shale, siltstone, and clay were muddy stream or lake
bottoms. The discontinuous ledges of conglomerate probably represent
gravel bars formed during flood stages or at places where the stream
currents were very swift.
Just rocks you may saybut look closely. A piece of sandstone
contains grains of sand that differ from each other in size, shape, and
composition. Frequently these characteristics point to the source of the
sandstone and tell something about the conditions at the time it was
deposited. Chunks of black material are examined closely and prove to be
charcoalcarbonized remains of plants.
Microscopic examination of clay fragments reveals shards of volcanic
glass and ash that speak of active volcanoes. Sometimes these clays bear
the carbonized imprints of delicate plants that long ago sank to the
bottom of some lake or stream where they were buried and fossilized.
The fossils themselves are most important in reconstructing
conditions of the past. We find the shells of fresh-water clams in the
sandstones with dinosaur bones. Crocodile bones are also common. We are
reasonably sure, then, that these deposits of sand and mud were formed
in rivers and lakes when the climate was mild.
We reason by analogy. For example, fossil plants and animals have
counterparts or descendants in the world of today. We assume, in the
absence of contrary evidence, that the fossil animals lived like their
present-day counterparts. Although no birds, mammals, or pterosaurs have
been found in this quarry, they were probably living here with the
dinosaurs. It is possible, in fact probable, that some modern animals
and plants live in different environments than did their Morrison
ancestors and relatives but we have no way of knowing which ones they
were. We can only take the data available, arrange them as logically as
possible, and continue the search for more. Some may scoff at such
methods of reasoning yet they do provide good results. What other
methods can be used when the world under investigation lies millions of
years in the past?
|